Add an instruction on how to format code with Prettier

**Which module(s) and week(s) does this change affect?**

Module(s): Onboarding
Week(s): 1

**What is the work that needs to be done?**

Adding a short explanation on how to format code with Prettier in the onboarding module.

**Why is this work important to do?**

It will give learners and trainees an understanding of how to use this extension and help volunteers review pull requests.

**Additional context**

learners and trainees